Contactless chip cards communicate with the reader without hard electrical connections. In this case, a distinction is drawn between passive chip cards and active chip cards. In the case of the “passive chip cards”, the power required for communication and data processing needs to be provided externally. The power is frequently provided by the reader in the form of an electromagnetic field, i.e. the power is obtained from the received signal.
FIG. 1 shows a principle which is known from the prior art for supplying power to a contactless chip card. The reader 1 uses the reader antenna 2 to generate an electromagnetic field. The electromagnetic coupling between the reader antenna 2 and the chip card antenna 3 is a coupled coil. The chip card antenna 3 has a resonance capacitor 4 connected in parallel with it whose capacitance is chosen such that, together with the coil inductance of the chip card antenna 3, a parallel resonance circuit is formed whose resonant frequency corresponds to the transmission frequency of the reader 1. A resonance rise in the parallel resonant circuit causes the induced voltage between RF1 and RF2 to reach a maximum. The induced voltage is rectified by a rectifier 5 and is smoothed by means of a smoothing capacitor 6, and is used to supply the chip 7 mounted on the chip card with the voltage RFVDD.
The voltage induced between RF1 and RF2 is highly dependent on the coupling factor of the reader antenna 2 and the chip card antenna 3. The coupling factor is determined by the distance between the chip card and the reader 1. At great distances, the coupling is weak, and only a low voltage is induced. At short distances, a resonance rise means that the induced voltage very quickly reaches high values which can damage the chip 7 as a result of overvoltage.
FIG. 2 shows another solution which is known from the prior art and in which the chip is protected against overvoltage. In this case, the arrangement from FIG. 1 is extended by a shunt rectifier 8, having a shunt element 9 and a shunt element actuation unit 10, which is connected in parallel with the nodes RF1 and RF2. If the voltage induced between RF1 and RF2 is too high, the shunt element actuation unit 10 is used to actuate the shunt element 9 such that the output of the shunt rectifier 8 is shorted to ground. Some of the current induced in the chip card antenna 3 is thus discharged by the shunt element 9 and thus stabilizes the voltage RFVDD for supplying the chip 7. The use of the additional, parallel-connected shunt rectifier 8 reduces the voltage ripple in the supply voltage RFVDD (WO 01/06630 A1).
FIG. 3 shows an embodiment of the rectifier 5 and shunt rectifier 8 which is known from the prior art. The connections RF1 and RF2 have an AC voltage applied to them. This voltage is rectified and is available as a DC voltage between RFVDD and ground. The rectifiers 5, 8 comprise four NMOS transistors T1 to T4 which are operated as diodes by connecting the gate connection to the source connection. The transistors T1 to T4 are connected up in the same way as the diodes in a full-wave bridge rectifier which is known from the prior art. In this case, two transistors from the four transistors are always on and two are always off.
For a positive half-cycle of the induced voltage, RF1 is greater than RF2. T3 and T2 are on, while T1 and T4 are off. The current flows from RF1 via T3 to RFVDD and through the load and via the ground and T2 back to RF2. For a negative half-cycle, RF2 is greater than RF1. T4 and T1 are on, while T3 and T2 are off. The current flows from RF2 via T4 to RFVDD and through the load and via the ground and T1 back to RF1. This interconnection means that the current always flows in the same direction through the load connected between RFVDD and ground, regardless of the polarity of the voltage which is present between RF1 and RF2.
FIG. 4 illustrates the load modulation principle which is known from the prior art. Load modulation can be used to transmit data from the chip card to the reader 1. A modulator is connected between the smoothed voltage RFVDD and ground. The modulator comprises a modulation resistor 11 and a modulation element 12. In this example, the modulation element 12 is in the form of a transistor. As a result of the modulation element 12 being turned on and off by the signal MOD, the electrical load in the chip card and hence also the voltage on the chip card antenna 3 change. Since the chip card antenna 3 is connected to the reader antenna 2 by means of an electromagnetic field, load changes also affect the antenna voltage of the reader 1. The antenna voltage of the reader 1 is modulated by the changes in the electrical load in the removed chip card. In this case, the ratio of the amplitudes of the antenna voltage when the modulation element 12 is off and on is called the depth of modulation. When the data to be transmitted from the chip card to the reader are applied as signal MOD, they control when the modulation element 12 is turned on and off. The resultant changes in the antenna voltage in the reader 1 then correspond to the data which are to be transmitted and can be demodulated. This form of data transmission is called load modulation. FIG. 4 also shows a voltage regulator 13 with a transistor, which is intended to minimize voltage fluctuations in the supply voltage VDD for the chip 7 which arise as a result of the load modulation.
FIG. 5 shows a load modulation embodiment which is likewise known from the prior art. In this case, the modulator is not connected to the rectified voltage RFVDD, as in FIG. 4, but rather directly to the voltage between RF1 and RF2 which is provided by the chip card antenna. In the case of this embodiment, the modulator comprises two modulation resistors 16 and 17 and two modulation elements 14 and 15 which are respectively connected to ground in series between RF1 and RF2. The modulation elements are again in the form of transistors. This arrangement can be used to modulate both positive and negative half-cycles of the induced antenna voltage with the signal MOD. Besides lower circuit complexity, this arrangement has the additional advantage over FIG. 4 that the modulation's disturbances on the supply voltage RFVDD for the chip are smaller. In this case, the smoothing capacitor 6 is not discharged by the modulation process, since the rectifier 4 allows a flow of current in one direction only, namely in the one which charges the smoothing capacitor 6. In addition to the modulation resistors 16 and 17, it is also possible to use modulation capacitances 18 and 19 connected in parallel therewith. Besides the amplitude modulation, the voltage on the reader antenna is then additionally phase modulated.
In the case of the solution shown in FIG. 2 for protecting against overvoltages, it is a drawback that the shunt rectifier 8 produces an increased chip area requirement. The shunt element 9 itself, generally a transistor, also requires additional chip area, because it needs to be proportioned to be large enough to be able to dissipate excess power and convert it to heat. Since a large chip area has a direct effect on cost, the solution which is known from the prior art is unsatisfactory in this respect.
A drawback of the load modulation solutions known from the prior art which are shown in FIGS. 4 and 5 is that the level of the voltage RFVDD which is required for supplying the chip 7 is not taken into account during the modulation. If the chip card is at a great distance from the reader 1, only a low voltage is induced in the chip card antenna 3. In the case of additional load modulation of this voltage, the minimum supply voltage VDD for the chip 7 which is required may be undershot. In this case, breaks appear in the power supply for the chip 7, and communication with the reader 1 is then no longer possible. To bridge these breaks in supply, it would be possible to increase the size of the smoothing capacitor 6. A drawback of this, however, is the additional need for expensive chip area and, in an arrangement as shown in FIG. 4, the restriction in the modulation dynamics as a result of the smoothing capacitor 6.
Another drawback of the previously known apparatuses from FIG. 4 and FIG. 5 is that additional chip area is required for the modulation elements 12, 14 and 15 themselves. In this case, the modulation element needs to be provided with appropriately large proportions for its contact resistance to become as low as possible. Another inadequacy of the known circuits for load modulation is that the depth of modulation is not predictable.